The present disclosure generally relates to wellbore cleanout and other remediation operations, and, more specifically, to methods for removing inorganic scale in the presence of a consolidated particulate pack.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, cleanout and other remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Inorganic scale deposition frequently occurs during the lifetime of a wellbore. Scale deposition can be undesirable due to its propensity to decrease production by lowering a subterranean formation's permeability and restricting other flow pathways within the wellbore, including within particulate-packed fractures. In addition to decreased production, fines leached from inorganic scale can be exceedingly damaging to wellbore equipment, such as through abrasion and seal degradation.
Although scale dissolution and removal operations may be performed periodically during the lifetime of a wellbore, the most desirable techniques for addressing scale seek to prevent or limit its deposition in the first place. For this purpose, a number of scale control additives have been developed and are commonly used in the art.
Scale deposition in a wellbore often occurs due to initial dissolution of a material (e.g., a subterranean formation matrix), followed by re-precipitation of a dense scale deposit upon exceeding the solubility limit of the dissolved material under the chemical and physical conditions present within the wellbore. The deposited scale can be the same material as that initially dissolved or a different material generated from a further chemical reaction or morphological change. Deposited scale can often be highly dense and have a crust-like shell, thereby providing a low contact surface area for promoting redissolution during a cleanout operation.
Inorganic scale deposits may be formed from precipitated metal salts, such as metal carbonates or metal sulfates. Additional inorganic materials may also be present in combination with precipitated metal salts. For example, in siliceous subterranean formations (e.g., sandstone or shale formations), siliceous materials such as silicates or aluminosilicates may be present in combination with inorganic salts within an inorganic scale. Siliceous scale deposits often require treatment with a hydrofluoric acid source to achieve dissolution of their siliceous material. Metal salts, in contrast, can often be dissolved with common organic acids and/or mineral acids other than hydrofluoric acid (e.g., hydrochloric acid). In a somewhat different approach, chelating agents or other ligands may promote dissolution of metal salts within an inorganic scale deposit through metal ion complexation. Many chelating agents contain multiple carboxylic acid groups, which in many cases are believed to be the active metal-complexing species. As used herein, the terms “complex,” “complexing,” “complexation” and other grammatical variants thereof will refer to the formation of a metal-ligand bond, such as through formation of a chelate.
Inorganic scale deposition can be especially problematic in the presence of a particulate pack, such as a proppant pack or a gravel pack. Particulate packs provide a large surface area upon which initial scale nucleation can commence and subsequent deposition can take place. Since the average spacing between particulates in particulate packs is relatively small, it can take relatively little scale deposition to significantly decrease the particulate pack's fluid permeability. Hence, descaling operations in the presence of a particulate pack can be highly desirable in order to sustain production from a wellbore.
Particulate packs often contain particulates that are consolidated with one another in order to retain the particulates in a set location within the wellbore. As used herein, the term “consolidated” refers to the adherence of a plurality of particulates to one another to form a coherent mass with retained permeability. In many instances, cured resins or like substances are used to achieve consolidation of particulates with one another.
Although a wide breadth of resins of varying structures and properties are known, a fair number of resins are unstable to varying degrees in the presence of acids. Such resins are referred to herein as “acid-unstable” resins. Both mineral and organic acids can lead to degradation of acid-unstable resins. Even the organic acid groups of many common chelating agents can instigate resin degradation under the extreme conditions present within a wellbore. Accordingly, it can be a very difficult task to remove inorganic scale in the presence of a resin-consolidated particulate pack, particularly when the inorganic scale is located within the particulate pack itself.
Although resin instability is undesirable in many instances, it can occasionally be useful to deconsolidate a particulate pack and remove it from a wellbore. For example, a remediation operation may need to be conducted without the particulate pack being held in place. Deconsolidation can be achieved through resin degradation, such as through treatment with an appropriate acid in the case of acid-unstable resins. Resin removal treatments in common use frequently utilize strong mineral acids that can present undesirable safety risks and environmental concerns.